1. Field of the Invention
The present invention relates to a VTR (Video Tape Recorder) signal processing circuit and, particularly, relates to a technique which is effectively used in a device including a frequency converter portion of a double main converter system which removes color cross-talk contained in a reproduced color under signal by using two frequency converter circuits and a delay circuit such as a CCD (Charge-Coupled Device).
2. Description of the Related Art
FIG. 5 is a block diagram of a frequency converter portion of a conventional double converter system of the prior art.
FIG. 6(a) is a waveform diagram of a conventional delaying operation of color under signal of the prior art when it operates stably.
FIG. 6(b) is a waveform diagram of a conventional delaying operation of color under signal of the prior art with corresponding to a variation of rotation speed of a motor.
In a home used VTR (Video Tape Recorder), a color video signal is recorded according to the color under system. In this system, a luminance signal is frequency-modulated, a color signal is frequency converted to a frequency band lower than the frequency-modulated luminance signal and the both are recorded on a slanted track of a magnetic tape by a rotary video head.
In a recent VTR, a guard-band-less system is used for increasing the recording density and cancel of cross-talk caused by the guard-band-less system is indispensable. The cancel of cross-talk is performed by providing azimuth angle to the video head. However, although the effect of azimuth angle is effective for a high frequency signal, it is less effective for low frequency signal. That is, the effect of cross-talk cancel is small for the color signal and, therefore, the phase shift color system (PS color system) or phase invert color system (PI color system) is employed.
The method for cancelling color cross-talk in the color under system will be described by using the NTSC format of the VHS system. A video recording track is recorded by repeating two channels, a channel 1 and a channel 2, alternately. In the VHS system NTSC format, a color under frequency is 40 times the horizontal scan frequency (40 f.sub.H). Therefore, the recording is performed by frequency converting a standard color signal frequency (color sub-carrier frequency) of 3.579545 MHz into 40 f.sub.H, about 629 kHz, with phase thereof in the channel 1 being advanced by 90 degrees every horizontal period and in the channel 2 being delayed by 90 degrees every horizontal period. With this phase shifting, a cross-talk component can be cancelled by, in a reverse conversion of the color signal of 629 kHz into that of 3.579545 MHz during reproduction, adding the color signal before delaying and the color signal delayed by a one horizontal period by using a delay element of one horizontal period.
When the delay element is one using a glass delay line, there is a problem since the glass delay line itself is a relatively large component part in mounting on a circuit board and is expensive. When the delay element is one using a CCD (Charge-Coupled Device), the CCD which operates with a clock 3 or 4 times the color sub-carrier frequency of 3.579545 MHz is required and further a phase adjustment to make a total delay amount of the CCD and a low-pass filter or a band-pass filter provided subsequently to the CCD exactly one horizontal period is necessary.
In view of this, a VTR signal processing circuit shown in FIG. 5, which has a simple construction and makes a reproducing frequency conversion possible while cancelling the color cross-talk, has been proposed. Taking a reproducing color signal processing circuit of a VTR of the VHS system of 3.58 NTSC system as an example, the color under signal having frequency of 40 fh, wherein fh stands for horizontal sync frequency, on one hand, is converted into a standard color signal by a frequency converter (1) circuit (main converter) 101. This converter circuit is a kind of the balanced modulator circuit for obtaining, when a frequency of a converted signal is sufficiently higher than a frequency of an input signal, a sum frequency component and a difference frequency component of the input signal frequency and the sufficiently high frequency. The color under signal is delayed by a CCD 103 by one horizontal period (two horizontal periods in the PAL system) and is frequency converted by the other frequency converter (2) circuit 102. A comb type filter is constituted by adding the standard color signal whose phase are made opposite by the two frequency converter circuits 101 and 102 by an adder circuit 104.
The carrier frequency of about 4.21 MHz for the above mentioned frequency conversion is produced by comparing in phase the color signal derived from the band-pass filter (BPF) 105 with a reference frequency signal produced by crystal oscillator circuit (VXO) 109 by a phase detector 108, producing an oscillation frequency signal (535 fh, about 8.42 MHz) which is twice the carrier frequency for the VTR recording and reproduction by controlling a voltage control oscillator (VCO) 107 according to a result of phase detection and dividing it by two by a frequency divider 111, and the above mentioned two frequency converted signals are made opposite by shifting the phase of the output of the frequency divider by 90 degrees. When the frequency converted signals are made in phase, the color cross-talk between adjacent tracks is cancelled by not addition but subtraction. Such double main converter system is proposed in Japanese Patent Application Laid-open No. H7-99671. In this double main converter system, the clock frequency of the delay circuit composed of CCD, etc., for delaying the color under signal may be low and can be realized without adjustment.
In such double converter system as mentioned above, there may be a possibility that an image quality is temporarily degraded or color is lost in relation to a response of a rotation speed of a drum motor mounted with a head when an operation is shifted to a special reproducing mode, wherein the special reproduction mode is mode shift to a search, slow, or still, etc. The reason for that is as follows: in the construction which uses a frequency which is twice the oscillation frequency of the VXO 109 obtained by a frequency multiplier circuit (x2) 110 as the clock signal of the pCD 103 for producing the delay signal of the above mentioned one horizontal period (1H), it is possible to obtain an exact delay time corresponding to the normal 1H when it operates stably as shown in FIG. 6(a). However, the phase is shifted correspondingly to a variation of rotation speed of a motor and the delay signal due to the CCD 103 which is kept the normal 1H, as shown in FIG. 6(b). With this phase shift, the cancelling effect of cross-talk from adjacent tracks is degraded causing the image quality to be degraded and, when the phase is inverted, the signal component is cancelled resulting in a color-less image.